Pipelines carrying heavy or crude oil need to be thermally insulated as heavy oil tends to solidify during transport from a subsea production well to the surface due to heat losses in the submerged pipeline. Thermal insulation is also required to avoid the formation of hydrates which can occur for certain crude oil compositions when the crude oil cools down, for example, when there is a breakdown in production flow rate.
Production lines which require a high level of thermal insulation typically use a double-walled pipe structure, for example a pipe-in-pipe system. A pipe-in-pipe system comprises an internal pipe within an external pipe separated by an annulus volume. In such a structure, the annular space can be filled with thermal insulation material. This structure has the advantage that the external pipe keeps the annular space dry and so, for example, in subsea pipelines, the thermal insulation material is protected from water. A further advantage of this structure is that the pressure in the annulus can be different from that outside the external pipe and that inside the internal pipe. This is important if the insulating material has a particular pressure requirement or if a vacuum or partial vacuum is to be used for insulating purposes. For example, the annulus can be at atmospheric pressure while the hydrostatic pressure experienced by the external (or carrier) pipe and the internal pressure of the fluid in the internal pipe (flowline) are different. Furthermore it is interesting to lower the pressure in the annulus in order to increase the thermal insulation performance.
One of the problems associated with such pipelines is that of safeguarding the annular space against the ingress of water, for example due to leaks in the external or carrier pipe. Water in the annular space will conduct heat from the inner flowline to the carrier pipe thus destroying the effectiveness of the insulation. This problem has been approached in prior art pipe-in-pipe systems by compartmentalising the annular space by means of permanent seals (GB 2 317 934, U.S. Pat. No. 2,930,407, WO 00/09926).
Such prior art seals or waterstops are useful for compartmentalisation of the annulus in the longitudinal direction so that it can remain partially dry in case of a leak in the carrier pipe. However, such an arrangement has a major drawback when it is desired to combine passive thermal insulation with an active heating system which may require the use of electrical cabling or hoses with heat transfer fluid. Waterstops create discontinuities and block passage of any equipment running along the length of the production line, such as that which may be required for an active heating system.